Applying materials on mesh models present in different blend files using python script
My first suggestion would be to avoid using bpy.ops and instead use code like http://web.purplefrog.com/~thoth/blender/python-cookbook/link-external-data-blocks.html which I have excerpted here:

import bpy


def library_get(name, data_blocks, libpath=None):
    """ Since it is possible for a .blend file to have several objects 
with the same name linked from different files, this routine lets us 
hunt down the right one. """
    for x in data_blocks:
        if x.name == name and x.library is not None:
            if libpath is None or libpath == x.library.filepath:
                return x
    return None


def addGroupToScene(grp, scn):
    for obj in grp.objects:
        scn.objects.link(obj)

#
#

fname = "//cube.blend"

print(fname)
with bpy.data.libraries.load(fname, link=True) as (src, dst):
    # src has many properties, each of which is a list of name strings.
    # dst is how we specify what we want to link from the library file.

    # let's link the "dazed" scene
    dst.scenes = ["dazed"]
    # and the "omnitile" object
    dst.objects = ["omnitile"]
    # and an object group
    dst.groups = ["frog"]

    # and just to be fancy, let's count up how many linkable things there are
    for dbt in dir(src):
        name_list = getattr(src, dbt)
        print("%s[%d]" % (dbt, len(name_list)))

# just because we linked an object doesn't mean that object is in any of our scenes.
omnitile = library_get("omnitile", bpy.data.objects)

scn = bpy.context.scene
try:
    scn.objects.link(omnitile)
    # now it is.
except:
    pass

# link all the objects from group "frogs" to the current scene
addGroupToScene(library_get("frogs", bpy.data.groups), scn)


Your version would probably have dst.materials = ["mainMaterial"]
and mat = library_get("mainMaterial", bpy.data.materials)